1. Technical Field
Provided are methods of forming resistive switching memory devices and arrays and storing N bits of data in resistive switching memory devices. Provided also are various structural features of resistive switching memory devices and arrays.
2. Description of the Related Art
Nonvolatile memory devices are used in systems in which persistent storage is required. For example, nonvolatile memory cards are used in digital cameras to store images and in digital music players to store audio data. Nonvolatile memory devices are also used to persistently store data in computer environments.
Electrically-erasable programmable read only memory (EEPROM) technology is often used to form and program nonvolatile memory devices. This type of nonvolatile memory contains floating gate transistors that can be selectively programmed or erased by application of suitable voltages to their terminals. As fabrication techniques improve, it is becoming possible to fabricate nonvolatile memory devices at increasingly smaller dimensions. However, as device dimensions shrink, scaling issues pose challenges for traditional nonvolatile memory technology. This has led to the investigation of other alternatives, including nonvolatile resistive switching memory technology.
Resistive Random Access Memory (RRAM or ReRAM) is often made by interconnecting a number of nonvolatile resistive switching memory cells into memory arrays, such as cross point ReRAM arrays, in order to increase the density of the resistive switching memory cells connected together in a memory chip. Nonvolatile resistive switching memory devices and systems are formed using bistable resistive switching memory cells. A current steering element, such as a diode, is often used to prevent cross-talk between the interconnected memory devices from affecting the stored data in other memory devices within the memory array. The diode only allows limited programming and erasing currents to pass through the resistive switching memory element in desired directions.
Each memory cell or device has a resistive switching memory element capable of being placed at a high resistance state (HRS, e.g., a logic “0” state) or a low resistance state (LRS, e.g., a logic “1” state) by applying suitable voltages or currents. For example, voltage pulses are generally used to switch the resistive switching memory element from one resistance state to the other. Nondestructive read and write operations are performed to ascertain the value of a data bit that is stored in a resistive switching memory device.
As each resistive switching memory element is only capable of being at two stable resistance states (HRS and LRS) to function in conjunction with one current steering element as a memory unit, only one (1) data bit per unit can be stored. Therefore, there is a need for resistive switching memory devices and arrays to be able to switch among multiple resistance states for storing more than 1 bit of data (e.g., multiple bits, N>1) in order to increase the amount of the data storage without the need to increase the size of a memory unit or the size of a memory array (and hence the size of a memory chip).